1. Field of the Invention
The present invention relates to improved lentiviral vectors, their production and their safe use in gene delivery and expression of desired transgenes in target cells.
2. Description of Related Art
Transfection of cells is an increasingly important method of delivering gene therapy and nucleic acid based treatment for a number of disorders. Transfection is the introduction of nucleic acids into recipient eukaryotic cells and the subsequent integration of the nucleic acid sequence into chromosomal DNA. Efficient transfection requires vectors, which facilitate the introduction of foreign nucleic acids into the desired cells, may provide mechanisms for chromosomal integration, and provide for the appropriate expression of the traits or proteins encoded by those nucleic acids. The design and construction of efficient, reliable, and safe vectors for cell transfection continues to be a substantial challenge to gene therapy and treatment methods.
Viruses of many types have formed the basis for vectors. Virus infection involves the introduction of the viral genome into the host cell. That property is co-opted for use as a gene delivery vehicle in viral based vectors. The viruses used are often derived from pathogenic viral species that already have many of the necessary traits and abilities to transfect cells. However, not all viruses will successfully transfect all cell types at all stages of the cell cycle. Thus, in the development of viral vectors, viral genomes are often modified to enhance their utility and effectiveness for introducing foreign gene constructs (transgenes) or other nucleic acids. At the same time, modifications may be introduced that reduce or eliminate their ability to cause disease.
Lentiviruses are a subgroup of retroviruses that can infect nondividing cells owing to the karyophilic properties of their preintegration complex, which allow for its active import through the nucleopore. Correspondingly, lentiviral vectors derived from human immunodeficiency virus type 1 (HIV-1) can mediate the efficient delivery, integration and long-term expression of transgenes into non-mitotic cells both in vitro and in vivo (Naldini et al., 1996a; Naldini et al., 1996b; Blomer et al., 1997). For example, HIV-based vectors can efficiently transduce human CD34+ hematopoietic cells in the absence of cytokine stimulation (Akkina et al., 1996; Sutton et al., 1998; Uchida et al., 1998; Miyoshi et al., 1999; Case et al., 1999), and these cells are capable of long-term engraftment in NOD/SCID mice (Miyoshi et al., 1999). Furthermore, bone marrow from these primary recipients can repopulate secondary mice with transduced cells, confirming the lentivector-mediated genetic modification of very primitive hematopoietic precursors, most probably bona fide stem cells. Since none of the other currently available gene delivery systems has such an ability, lentiviral vectors provide a previously unexplored basis for the study of hematopoiesis and similar phenomena, and for the gene therapy of inherited and acquired disorders via the genetic modification of human stem cells (HCLs).
This important capability is subject to significant biosafety concerns (Akkina et al., 1996; Sutton et al., 1998; Uchida et al., 1998). The accidental generation of replication-competent recombinants (RCRs) during the production of lentiviral vector stocks represents one of the major problems to be solved before lentiviral vectors can be considered for human gene therapy.
In the retroviral genome, a single RNA molecule that also contains all the necessary cis-acting elements carries all the coding sequences. Biosafety of a vector production system is therefore best achieved by distributing the sequences encoding its various components over as many independent units as possible, to maximize the number of crossovers that would be required to re-create an RCR. Lentivector particles are generated by co-expressing the virion packaging elements and the vector genome in host producer cells, e.g. 293 human embryonic kidney cells. In the case of HIV-1-based vectors, the core and enzymatic components of the virion come from HIV-1, while the envelope protein is derived from a heterologous virus, most often VSV due to the high stability and broad tropism of its G protein. The genomic complexity of HIV, where a whole set of genes encodes virulence factors essential for pathogenesis but dispensable for transferring the virus genetic cargo, substantially aids the development of clinically acceptable vector systems.
Multiply attenuated packaging systems typically now comprise only three of the nine genes of HIV-1: gag, encoding the virion main structural proteins, pol, responsible for the retrovirus-specific enzymes, and rev, which encodes a post-transcriptional regulator necessary for efficient gag and pol expression (Dull, et al., 1998). From such an extensively deleted packaging system, the parental virus cannot be reconstituted, since some 60% of its genome has been completely eliminated. In one version of an HIV-based packaging system, Gag/Pol, Rev, VSV G and the vector are produced from four separate DNA units. Also, the overlap between vector and helper sequences has been reduced to a few tens of nucleotides so that opportunities for homologous recombination are minimized.
HIV type 1 (HIV-1) based vector particles may be generated by co-expressing the virion packaging elements and the vector genome in a so-called producer cell, e.g. 293T human enbryonic kidney cells. These cells may be transiently transfected with a number of plasmids. Typically from three to four plasmids are employed, but the number may be greater depending upon the degree to which the lentiviral components are broken up into separate units. Generally, one plasmid encodes the core and enzymatic components of the virion, derived from HIV-1. This plasmid is termed the packaging plasmid. Another plasmid encodes the envelope protein(s), most commonly the G protein of vesicular stomatitis virus (VSV G) because of its high stability and broad tropism. This plasmid may be termed the envelope expression plasmid. Yet another plasmid encodes the genome to be transferred to the target cell, that is, the vector itself, and is called the transfer vector. Recombinant viruses with titers of several millions of transducing units per milliliter (TU/ml) can be generated by this technique and variants thereof. After ultracentrifugation concentrated stocks of approximately 109 TU/ml can be obtained.
The vector itself is the only genetic material transferred to the target cells. It typically comprises the transgene cassette flanked by cis-acting elements necessary for its encapsidation, reverse transcription, nuclear import and integration. As has been previously done with oncoretroviral vectors, lentiviral vectors have been made that are “self-inactivating” in that they lose the transcriptional capacity of the viral long terminal repeat (LTR) once transferred to target cells (Zufferey, et al. 1998). This modification further reduces the risk of emergence of replication competent recombinants (RCR) and avoids problems linked to promoter interference.
Nevertheless, experience with retroviral vectors demonstrates that the emergence of a replication-competent retrovirus (RCR) is possible, although a rare event even when vectors are produced by stable packaging cell lines and components designed to provide high safety. The pathogenic potential of RCRs is demonstrated by the induction of cancer in monkeys injected with contaminated oncoretroviral vector stocks. Consequently, the administration of retroviral vectors to human patients is authorized only if the presence of contaminant RCRs has been excluded by a test sensitive enough to detect a single RCR in an aliquot equal to 5% of the dose actually used. Creating highly safe vectors is clearly important when doses equal or superior to 1010 transducing units may be necessary to reach therapeutic efficiency.
There is therefore a significant need to develop improved lentiviruses for use as transducing vectors capable of effectively transducing cells and expressing desired transgenes at high levels while meeting biosafety requirements. Currently available lentiviral vector production systems rely on the expression of packaging and vector elements either by transient transfection or in stable cell lines. Deletion of non-essential genes from the parental virus and splitting of the vector system components on separate DNA units act to help minimize the risk of emergence of RCRs. Greatest safety is achieved with the fewest, or, ideally, with zero RCR occurrence in vector production. The present invention utilizes specific changes in the packaging and vector system components, their methods of production and their methods of use in order to further reduce or eliminate the occurrence of RCR.